A variety of experiments in Nuclear Magnetic Resonance (NMR) could benefit from miniaturization of the detector coil. When samples are mass-limited, reducing the detection volume to match the sample size offers enhanced Signal-to-Noise-Ratio (SNR) performance. To date, efforts made to perfect high-resolution spectroscopy in very small coils have not been suitably effective [1-9], particularly in developing portable detection devices. The integration of NMR with separation techniques such as chromatography (e.g., [10]) or capillary electrophoresis [11,12] proceeds more naturally when the NMR detection volume can be made compatible with the very small sample volumes and fluid handling tubing typical of the separation step. Researchers have also sought the integration of NMR with microfluidic lab-on-a-chip devices, in which case the NMR detector coil is often formed in a lithographic-type process [13,14]. Very small coils have also been recognized as potential platforms for studying or imaging extremely small objects, even single cells [15-19]. All of the above applications have typically been implemented on traditional, high-field superconducting NMR magnets. These magnets provide the best route toward maximal SNR performance, have very stable and homogeneous fields, and are readily available in most NMR research groups.
A fully miniaturized NMR device would be based on a very small permanent magnet. Modern permanent magnet designs result in compact, lightweight devices operating at fields of 1-2 Tesla [24]. Although higher fields could be attained in a small superconducting magnet, the fragility and high maintenance needs of that magnet technology make it unattractive for portable or industrial applications. However, the low field of the permanent magnet presents a number challenges. First, the signal to noise ratio (SNR) in NMR is usually proportional to ω07/4=(γB0)7/4, where ω0 is the NMR frequency and B0 is the field strength [25]. SNR is already at a premium for micro-scale sample volumes, so detector circuits and electronics must be optimally efficient. At low frequencies, the electrical skin depth is no longer small compared to wire diameters, a regime in which optimal NMR SNR performance has not been explored experimentally, to our knowledge. More subtly, a low field and correspondingly low NMR frequency makes it difficult to construct an electrically resonant LC probe circuit from the very-low-inductance sample coil. We have recently proposed and implemented [26] a solution to this problem that introduces a large, fixed-value, auxiliary inductor to eliminate the need for a very large tuning capacitance, which would be awkward to use and would be inconsistent with the design goals of a practical compact system. Understanding and meeting the particular challenges of operating very small NMR detector coils in low-field permanent magnets is a crucial step toward the realization of very small, simple, and inexpensive NMR systems that are capable enough to enable new applications.
While some progress has been made in developing such portable microcoil-based NMR systems [26], improved devices that provide improved SNR, line width performance, and other benefits would be of great value to the art. Such improvements not only will greatly improve detection capabilities, but also would allow further reductions in sample volume and further miniaturization of the device.